|Turning | Lathe Machines|
Aronson, Robert B
Turning machines are doing much more
No machine-tool type has reached beyond its initial simple purpose like the lathe. With the addition of extra turning spindles, and live spindles for drilling, milling, and even grinding, turning machines have become turning centers. At the same time, chucks, robots, and internal handling systems have made the lathe a system that's able to feed itself and provide complex internal part handling.
At the leading edge, the need for greater speed and precision, with reduced part handling, is driving designs. At the same time, there is a significant group that is doing well with a "just enough technology to get by" attitude.
Mighty Multitasking. "There has been an evolution of the multitasking market,"says Chuck Birle, Marketing Vice President of Mazak's Performance Division (Florence KY).
"Machines such as our Integrexes combine turning, and milling/ drilling/tapping and probing. The benefits of multitasking turning include: reduced workpiece setups, reduced tooling and fixturing, increased workpiece accuracy and, probably most important, it offers users an even throughput. Parts, or sets of parts can ship as they come off the machine."
"In the evolution of turning machines, we had turning centers with single-point turning, then automated loading and unloading. Next came the idea of a turning center that could do the same work as a machining center. A lot of builders moved into multitasking by making modifications to existing two-axis lathe designs, such as adding live milling heads. This approach had limitations: tooling interference, programming difficulties with the additional machine axis, and most important, severe limitations to the machining abilities. With our Integrex design, we took a "white paper" approach. As a result, almost anything you can do on a machining center, you can do on an Integrex.
"Initially, with our Integrexes we saw turning operations as the dominant process, now things are shifting away from turning-dominated operations. One company that bought an Integrex hardly does any turning at all. Milling dominates the machine's operation. This operator takes advantage of the fixturing reduction the Integrex provides. Instead of elaborate, inflexible, and expensive machining center fixtures, fixtures are now just chuck jaws. Instead of turning and machining-center tooling being scattered around the shop, tooling is resident in the ATC behind the Integrex.
"By moving the machine's cutting operations around the workpiece, rather than moving the workpiece around the shop to various machines for different operations, crucial workpiece datum's are not lost, they stay digitally contained within the machine's axis envelope. With larger workpieces weighing hundreds of pounds, safety is a big factor since the workpiece can be automatically loaded and unloaded. In high tech applications, such as medical, semiconductor, or aerospace, reduced handling also means reduced workpiece marring.
"Once companies realize the value of multitasking, they want to put more and more part numbers on the machine, making it easier to justify than they originally anticipated. Now, with Integrexes having Y and B axis capabilities, shops are able to machine the entire part complete in one handling."
"In addition," notes Dick Lewis of Hardinge (Elmira, NY), "there is more demand for working with near-net-shape parts, and this requires minimal setup to be profitable."
"We're doing a lot of multitasking and can mill, drill, and turn in one machine. This allows us to make complex parts with fewer setups," says Bob Lewis, senior manager of applications for Okuma America Corp. (Charlotte, NC). "If you are going to do four operations, you have to queue them up waiting for machine time. So queue time is often longer than operating time. We want to avoid `sleeping inventory' that is, parts sitting around waiting for the next step in the process. With a multitask machine, you put in raw stock and take it out a complete part. There is no queue time. We are not doing any grinding because we are not comfortable with the effects of grinding dust on the machine."
"Naturally, one machine's design can't do it all," says Mazak's Birle. "Workpiece flow is king. In some very high volume applications, you may want to isolate individual machining processes and pass the part from machine to machine.
Multitasking fits well in lower to medium-volume part runs. But this is changing since the newer, smaller Integrexes with fast machining and chip-to-chip tool-change times make sense for even some higher volume applications. With multitasking, there are so many machine axes to keep track of (C, X, Z, Y and now B) that a strong conversational control is needed to reduce programming times."
General wants. "The early CNC multispeed machines were slow and cumbersome," says Mark Smith, vice president, Schutte MSA (Jackson, MD. "But modern CNC, such as Schutle's virtual CAM control used in the S-32PC machine, provides the speed and control to handle the larger number of axes. Designs now range from commodity single-spindle lathes to multispindle machines.
"Our market is at the high-precision end making high-volume products. We have traditional camoperated machines for long runs and multispindle automatics for lower-volume production of precision components."
The newest Schutte machines are CNC units with as many as 30 programmable axes on six spindles that can mill, drill, and do ec cent ric work. The S32PS Camless Electronic Control system collects data from typical multispindle layouts with parallel processing software and prompt screens. The control then calculates and coordinates the stroke and synchronization of a maximum of 30 axes. Little, if any, programming is needed, so the system can be readily integrated into conventional multispindle shops.
"The mill-turn machine is drawing a lot of interest, especially on larger machines where pallet shuttles and other material-handling systems are not cost effective or there are space problems," comments Ken Campshure, Giddings & Lewis (Fond du Lac, WI). "We make parts more than 84" [2.1 m] long, so you want to leave the workpiece in the machine for as many operations as possible. Another trend is to get more Y-axis motion. Some have that motion built into the machine, and others have an attachment on the ram.
"There is some interest in machines with grinding capability, particularly in the aerospace market, but that's limited. Normally, in a machine tool, the bearing spindle and table are not of the same quality as that needed for a grinding machine."
Simplicity is important. "At Bridgeport we see strong sales in the low end of the market," says Steve Miller, lathe product manager for Bridgeport Machines, Inc. (Bridgeport, CT). These buyers come from three areas.
People just getting into turning who want a machine that is easy to use, but one that has the potential to carry high-tech options.
Those with a severe shortage of trained turning operators who can't afford to risk a high-cost machine.
Those with a highly variable production volume who will buy three or more smaller machines instead of one massive high-production unit. That way, during low- volume demand, one machine runs and two are idle, but all three run when demand is higher.
"We offer machines that can be manual or automatic," says Miller. "For example, our EZPath III operates manually, as a semiautomatic, or a full automatic. As users want more sophistication, they can add accessories such as bolt-on CNC turrets or power chucks.
"Our designs have to take into account that there are fewer and fewer operators familiar with conventional flat-bed turning."
Manual/automatic combination lathes are popular for most conventional production. The next level employs automated accessories such as bolt-on CNC turrets, power chucks, and workholders with either air or manual control.
Heavy gripping. "We are looking at higher chuck speeds, more than 1.5 mile/min [2.4 km/min] at the chuck OD. You have to worry about centrifugal force on the chuck and you usually need a counter-centrifugal chuck, which is kind of expensive," explains G & L's Campshure.
Clever machine design does no good if the part is not securely held. "In our design, we counterbalance on the face of the three-jaw chuck," says Steve Ennis, sales engineer for Goss and DeLeeuw Machine Co. (Kensington, CT).
"We counterweight on the chuck face, not internally," explains Ennis. "With the internal design you are limited in the amount of counter mass you can fit in. Room is limited. We mount two sets of jaws: regular top jaws and low-profile, pie-shaped jaws, which we call balance jaws. With this arrangement, each top jaw has a balance jaw 180 away. The two are connected within the chuck. As the chuck spins, the top jaw tries to pull away from part while the balance jaw pulls in the opposite direction, negating the effects of centrifugal force. When you change jaw mass or design you can accommodate the increased forces by moving the balance jaw."
The Goss and DeLeeuw chuck reportedly doesn't lose any holding force at speed. For example, a standard 8' (200 mm), chuck works well at 5000 rpm and the high-production version can run at 9000 rpm. Actuation is by air or hydraulics. The completely sealed chuck is maintenance free with the lube sealed in. Jaws are easily changed and give highly repeatability.
Too many chips. Chip handling continues to grow as a problem. "It's bigger than we initially expected," says Okuma's Lewis, "because faster speeds mean more chips. In addition, there is more fluid to filter, and we need to find a way to get the chips out. Conveyors now have to have filtering capabilities, you can't just let the coolant run off. People are going to high-pressure coolant systems to move the chips." According to Yusuf Venjara, engineering manager for Hitachi Seiki (Congers, NY), "Pressures up to 1000 psi [6.9 MPa] are often needed because faster speed means more chips, and it's important to keep the cutting surface clean. With highpressure coolant, part making that formerly required four minutes can now be done in 40 sec. In addition, filtration is important."
"Customers don't want chip conveyers in the floor," explains G&L's Campshure, "because they want to be able to move the machine. We are responding by putting chip conveyors on the front of our machines when practical. In addition, we need to keep chips where they belong, so we give extra attention to enclosures."
Speed penalties. High speed is not the answer to everything. It depends on the application. "When you increase speed you sometimes sacrifice horsepower and torque. You can't always have it both ways," says Dick Lewis of Hardinge.
Campshure of G&L agrees, "Although high spindle speed gives you higher throughput, you have to pay for it in more expensive equipment. But customers are asking for higher traverse rates, faster tool change, and increased swing capacity for a given table size. For example, with a 36' (914-mm) diam table, the user wants a swing of 54 or 60" (1.4-1.5 m). They are not so much concerned about weight capacity, but size."
Big job spread "At the low end of the line, we have the simple, two-axis machine," explains Venjara of Hitachi Seiki "If you go to three or four axes, the tool has to be modified to machine with a fourth axis subspindle and toolchanger. The next step up is a move to a more complex machine like our Hicell.
"Our control system answers the need for the supervisor who wants more information and does not want to go to the floor and find out what's going on. We want to connect with the Ethernet and PCs. The next frontier coming is the phasing out of RS 232 and the introduction of the Ethernet and other proprietary systems like Mazak's fusion control system.
"You can't buy a low-cost machine and expect to get the work done, you still have to consider setup. We are working to see how we can use our Flex Link design to further reduce setup time.
"More integration is the next step in turning centers. Expect faster tool changes and a triple-station turret that provides turning, milling, and a station for specials. With this configuration, the part passes from station to station. The turning center is approaching the capability of a machining center," concludes Venjara.
"A trend that started several years ago is to first buy equipment for the simplest work, then move up as needed," says Lewis of Hardinge. "At Hardinge, we recognize the need for multiple product levels in our turning machines to address the entire market. We start with our economical Cobra lathes and move up to our newest machine, the Conquest TwinTurn 65, which is a fullfunctional twin-spindle, twin-turret turning center. In between, we offer gang-tool machines, Swiss machines, vertical turning lathes, and the flagship of our turning line, the Conquest T-series machines. We also recently introduced the GT Autoload, which features an automation system integrated with our GT gang-tool machine."
Another example of ever-expanding lathe offerings in the industry are the Hawk CNC turning centers from Cincinati Machine (Cincinnati). The company now offers the 6" (152-mm) TC-150, 8" (203 mm) TC-200 and 10" (254 mm) TC-250. They are all controlled by the PC-based Acramatic 2100 CNC.
On the other hand, it's possible to modify existing designs to meet fresh requirements. For example Haas (Oxnard, CA) took their proven SL30 lathe with programmable tailstock and added the spindle and gearbox from the larger SL-40. The result was the SL 30BB midsize CNC lathe with larger capacity.
This unit has a maximum cutting diameter of 14.5" (370 mm) and 30" (760 mm) swing.
Haas offers three basic models in their SL series to meet customer demand. The SL 20, SL 30 and SL 40 with 20, 30 and 40" (0.5, 0.8, and 1-m) swings respectively. Cutting capacities range from 10 X 20" (254 X 508 mm) to 25 X 44" (254 x 510 mm).
Watch out for the heat. "One way we improve accuracy is by doing a better job of thermal compensation," says Lewis of Okuma. "We monitor the heat better and know what to do with it. The improved accuracy means we are taking over more grinding jobs. Five years ago, one in 20 jobs was hard turning. Now it's five of 20, especially among auto products."
Saginaw Machine Systems, Inc. (Troy, MI) has been looking into ways of improving machine tool performance, particularly in the area of thermal compensation. According to Gerald Romito, vice president of technology, "The project is doing well and we currently have eight vertical lathes from our Crusader line in the field." To set up the system, they use probes positioned at key locations on the machine to develop a thermal map. Then they use this information to introduce compensating factors into the lathe's control.
"The key to making it work is the location of six thermal sensors that look at spindle growth, and X and Y-axis growth. We have held eight microns over 24 hours though a varying cycle that included cold start, cutting, rest, then cutting." Other trends include:
No more holes. Okumas's Lewis notes that installation is also changing. "No one wants to dig a hole in the shop floor any more. They want to have movable machines so they can reconfigure in a short time. We avoid the problem by making the entire floor 24" (610 mm) thick so we can put machines anywhere we want them."
Internal hands. A lot of the part handling goes on inside the enclosure after the part making starts. Lathes use a variety of robotic systems. Some have gantry robots to move parts, while others have stand-alone or built-in robots.
"We use robots in about 20% of our machines for automated loading, either gantry or stand alone," says Lewis.
Tighter specs. "The area in which we are seeing the most change is accuracy," Lewis states. "We now offer a total tolerance of 25 microns."
Then there is a push for hardturning applications, because people want to eliminate grinding.
Loading is important. "For parts from 6 to 8 lb [2.7-3.6 kg], automatic loading is usually practical," explains Vanjara of Hitachi Seiki. "Over that weight, the cost of the loader relative to the turning machine increases substantially. When part weight gets into the 20-30 lb (9-13.6 kg) range, loaders cost more than the machine."
Copyright Society of Manufacturing Engineers Jun 1999
Tags: CNC Machining Machining Technique Machine Tool Lathe Turning Lathe Machines
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